In the manufacture of tires having white letters or white stripes on their sidewalls, the usual practice is to mold the tire with a thin layer of black rubber covering the white letters or stripes. This black rubber must then be removed to expose the white material that lies underneath. To accomplish that task, the tire is usually mounted on the chuck of a rotatable spindle and a buffing wheel is moved into position next to the letters or striping on the sidewall. The tire is then rotated and the buffing wheel is moved toward the annular portion of the sidewall having the letters or stripes to be finished. The grinding wheel is gradually moved inwardly against the tire surface, until all portions of the letter or stripes around the circumference of the sidewall have had their black cover layer removed and have been properly finished to the appearance standards of the manufacturer.
Often, black sidewall tires, particularly high performance tires, are finished using the same buffing procedure and apparatus as used on white sidewall tires. Of course, in the case of black sidewall tires, the only purpose of that buffing is to improve the finish and appearance of certain portions of the tire sidewall, usually the letters that spell out the brand name of the tire.
One problem experienced with this conventional method and apparatus for finishing tire sidewalls is that the sidewall of tires have slight variations about their circumference, when compared to the plane of the tire's rotation. The buffing wheel does not take this dimensional variation of the tire into account while it is grinding the letters or stripes. Consequently, the letters or the stripes have their surfaces ground so that they lie in a common plane perpendicular to the axis of rotation of tire, while the adjacent unground portions of the tire sidewall vary with respect to that plane. Thus, the letter or stripes end up raised or indented at different levels above or below the tire surface. Normally, such differences are not enough to attract the attention of most observers, but they can make the finishing of some tires difficult. Also, the sidewall surfaces of tires that require buffing must usually be raised above the sidewalls of the tires, rather than indented, because of the danger of the buffing wheel cutting too close to the tire cords at the portions of the sidewall that protrude the most from the plane of rotation of the tire. It would save rubber if the portions of the sidewall to be buffed were indented rather than raised with respect to other portions of the sidewall.
The present invention solves the foregoing problem of removing material from a portion of a tire sidewall by mounting a sensor on the carriage that carries the grinding wheel or other cutting device. The sensor measures the distance of between the cutting device and a portion of the tire sidewall adjacent to the portion of the sidewall from which material is to be removed. As the tire rotates while the cutting device is removing material, the carriage moves in response to the sensor's measurement toward or away from the tire sidewall, so that the cutting device removes material at a constant level relative to the adjacent portion of the sidewall.
The treads of tires are frequently buffed to correct minor force variations and out-of-roundness. Machines that do such buffing have sensors which measure the tire's runout (roundness). However, those sensors are usually mounted separately from the carriage holding the grinding wheel, for instance the sensor 85 in FIG. 3 of U.S. Pat. No. 3,724,137 to Hofelt and Corl. Also, such sensors measure the tire runout on the same portion of the tire surface to be buffed. Their purpose is to detect those places on the circumference of the tire that protrude the most from the axis of the tire so that they can be buffed by the separately mounted buffing device. The apparatus and method of the present invention differs from such tread buffing systems, in that the sensor is mounted on the same carriage that carries the buffing device and is positioned to detect the circumferential variations of the tire surface on a portion laterally adjacent to the portion to be buffed. The buffing device does not remove material from the portion of the tire measured by the sensor, nor does it attempt to correct the variations measured by the sensor. Instead, the sensor feeds its measured surface variations back to a computer which adjusts the movement of the carriage on which the sensor is mounted, so that the carriage and a buffing device on the carriage conforms to those variations, as it removes material from another portion of the tire surface.
U.S. Pat. No. 4,084,350 to Ongaro shows a force and runout correction machine for tire treads of a different design. In FIG. 3, a sensor 126 is mounted on the same carriage as a buffing wheel 114. However, this sensor, like that of U.S. Pat. No. 3,724,137, is positioned to measure the runout of the same circumferential stripe on the tire tread that is to be cut by the buffing wheel. As described in column 9, lines 52 to 68 of the patent, the purpose of the sensor is to detect the high points on the portion of the tire to be buffed so that the buffing wheel can be initially spaced at a safe distance away from those high points. Also, according to column 13, lines 47 to 68, the runout variations measured by the sensor are used to determine whether they are within the limits such that it is appropriate to buff the tire. There is no feedback between the sensor and the carriage holding the buffing wheel that enables the buffing wheel to follow the runout variations measured by the sensor, as there is in the method and apparatus of the present invention.